Radio-Frequency Identification (RFID) technology has become widely used in virtually every industry, including transportation, manufacturing, waste management, postal tracking, airline baggage reconciliation, and highway toll management. RFID systems are often used to prevent unauthorized removal of articles from a protected area, such as a library or retail store.
An RFID system often includes an interrogation zone or corridor located near the exit of a protected area for detection of RFID tags attached to the articles to be protected. Each tag usually includes information that uniquely identifies the article to which it is affixed. The article may be a book, a manufactured item, a vehicle, an animal or individual, or virtually any other tangible article. Additional data as required by the particular application may also be provided for the article.
To detect a tag, the RF reader outputs RF signals through the antenna to create an electromagnetic field within the interrogation corridor. The field activates tags within the corridor. In turn, the tags produce a characteristic response. In particular, once activated, the tags communicate using a pre-defined protocol, allowing the RFID reader to receive the identifying information from one or more tags in the corridor. If the communication indicates that removal of an article has not been authorized, the RFID system initiates some appropriate security action, such as sounding an audible alarm, locking an exit gate, and the like.
Most methods of determining whether articles present in the interrogation corridor have been checked out depend upon first individually detecting and identifying each tag in the field, and then checking determining the status of the articles associated with the identified tags. Some methods, for example, involve determining a serial number for each tag, and then accessing a database to determine the status of the article associated with the identified serial number. Other techniques require issuing commands to the identified tags once the serial number has been determined.
This process can be time-consuming, especially if several tags exist in the field. For example, in order to obtain a complete tag serial number, only one tag can respond at a time. If more than one tag responds at a time, a collision occurs, the data received may be invalid, and neither tag's serial number can be obtained. To deal with this, some systems use an anti-collision process, which requires each tag to respond in a different time slot until all tags are heard. This added delay is undesirable in an exit control system because patrons are in the interrogation corridor for a very short period of time. Also, each patron can be carrying multiple books. The time required to determine whether every one of the books is checked-out is often much longer than the time a patron spends in the corridor.